The present invention relates to a fine yellow composite iron oxide hydroxide pigment, and a paint or a resin composition using the pigment, and more particularly, to a fine yellow composite iron oxide hydroxide pigment which are excellent in transparency and heat resistance, a paint or a resin composition using the pigment, and a process for producing the pigment.
Iron oxide hydroxide particles have been widely known as a yellow pigment because these particles can exhibit a yellow color, and have been used in many applications such as coloring of paints, printing inks, plastics, films, building materials, cosmetics and the like.
Among the iron oxide hydroxide particles, by using those having a particle size of less than 0.1 .mu.m, the formed coating film is transparent to visible light and, therefore, such particles are useful as a transparent yellow iron oxide hydroxide pigment.
The yellow iron oxide hydroxide pigment composed of iron oxide hydroxide fine particles having a particle size of less than 0.1 .mu.m (hereinafter referred to merely as "fine yellow iron oxide pigment") are deteriorated in dispersibility in vehicles or resin composition due to the reduction of the particle size thereof. Therefore, a coating film or a resin composition using the fine yellow iron oxide hydroxide pigment is unsatisfactory in transparency, and is also deteriorated in heat resistance.
Namely, the fine yellow iron oxide hydroxide pigment has a high surface energy and tends to be agglomerated due to the reduction of the particle size thereof, so that it is difficult to disperse the pigment in vehicles. Therefore, a coating film formed from such an insufficient dispersion cannot show a sufficient transparency since the fine pigment is agglomerated into coarse particles.
Consequently, it has been strongly demanded to improve not only the dispersibility of the fine yellow iron oxide hydroxide pigment in vehicles or resins but also the transparency of the pigment itself.
Further, since the iron oxide hydroxide particles has a chemical composition: Fe.sub.2 O.sub.3.H.sub.2 O (=2FeOOH), i.e., contain crystallization water therein, the dehydration thereof is generally initiated at about 200.degree. C. as the heating temperature is elevated. When the heating temperature is further elevated, the iron oxide hydroxide particles are transformed into reddish brown hematite (.alpha.-Fe.sub.2 O.sub.3) at about 230.degree. C. Thus, the iron oxide hydroxide particles per se are deteriorated in heat resistance. Especially, in the case of fine iron oxide hydroxide particles, the dehydration-initiating temperature tends to be further decreased due to a large specific surface area thereof. For this reason, it is difficult to apply the fine yellow iron oxide hydroxide pigment to thermoplastic resins such as polyethylene, polypropylene, styrene polymers, polyamides, polyolefins or ABS resins which are usually molded at as high a temperature as not less than 200.degree. C. Consequently, it has been strongly demanded to enhance the heat resistance of the fine yellow iron oxide hydroxide pigment.
Hitherto, in order to improve a weather resistance or a heat resistance of such a fine yellow iron oxide hydroxide pigment, there have been proposed and attempted various methods, e.g., a method of producing a transparent metal oxide comprising adding a surfactant to a sol composed of a hydrated metal oxide having an average particle size of not more than 300 .ANG., or agglomerating the sol by first adding aluminum ions and then the surfactant thereto (Japanese Patent Publication (KOKOKU) No. 6-2562(1994)); a method of forming a solid solution of AlOOH in each yellow iron oxide particle in order to enhance a heat resistance thereof (Japanese Patent Publication (KOKOKU) No. 55-8462(1980)); or the like.
In EP 0887387 A2, there have been proposed yellow iron oxide hydroxide particles comprising iron oxide hydroxide particle as Et core, composite oxide hydroxide of Fe and Al deposited on surface of the iron oxide hydroxide particle as a core, and optionally a hydroxide of aluminum covered on a surface of the composite oxide hydroxide, wherein the amount of Al in the composite oxide hydroxide is 0.1 to 10% by weight (calculated as Al) based on the weight of the iron oxide hydroxide particle as a core, the amount of Fe in the composite oxide hydroxide is 0.1 to 50% by weight (calculated as Fe) based on the weight of the iron oxide hydroxide particle as a core and the ratio of Al to Fe in the composite oxide hydroxide is 1:0.5 to 1:20 (0.05:1 to 2:1) (calculated as Al and Fe atoms); and having an average major axial diameter of 0.1 to 1.0 .mu.m, an average minor axial diameter of 0.02 to 0.10 .mu.m, an aspect ratio (major axial diameter/minor axial diameter) of 2:1 to 20:1, a BET specific surface area of 10 to 180 m.sup.2 /g and a heat-resistance temperature of not less than 265.degree. C.
At the present time, it has been most strongly demanded to provide a fine yellow iron oxide hydroxide pigment which are excellent in both transparency and heat resistance. However, such fine yellow iron oxide hydroxide pigment which can fulfill these properties cannot be obtained yet.
That is, the surfaces of metal oxide particles in the above-mentioned Japanese Patent Publication (KOKOKU) No. 6-2562(1994), are coated with aluminum oxide. However, as shown in Comparative Examples hereinafter, although such particles have been improved in transparency, the heat resistance thereof is still unsatisfactory.
The yellow iron oxide pigment described in Japanese Patent Publication (KOKOKU) No. 55-8462(1980) contains AlOOH in the form of a solid solution. Therefore, although such a pigment shows an excellent heat resistance, the transparency thereof is still unsatisfactory as also shown in Comparative Examples hereinafter.
Further, the yellow iron oxide hydroxide particles proposed in EP 0887387 A2, are composed of iron oxide hydroxide as a core particle, and have an average major axial diameter of 0.1 to 1.0 .mu.m. However, as shown in Comparative Examples hereinafter, the transparency of these particles is still unsatisfactory.
As a result of the present inventors' earnest studies for solving the above-mentioned problems, it has been found that by adding an aluminum compound and a ferrous salt compound to a water suspension of fine goethite particles which contain aluminum inside thereof in an amount of 0.05 to 50% by weight (calculated as Al) and have an average major axial diameter of not less than 0.005 .mu.m and less than 0.1 .mu.m, while stirring; and then passing an oxygen-containing gas through the resultant dispersion to coat a composite oxide hydroxide of Fe and Al on the surfaces of the fine goethite particles containing aluminum inside thereof, the obtained composite iron oxide hydroxide particles are useful as a fine yellow pigment which are excellent in both transparency and heat resistance. The present invention has been attained on the basis of the finding.